This invention relates to printing an image with multiple passes of a printing mechanism and more particularly relates to securely printing a postal indicia image utilizing multiple passes of a printing mechanism relative to a mailpiece.
Traditional postage meters imprint an indicia on a mailpiece or a label to be subsequently placed on a mailpiece as evidence that postage has been paid. These traditional postage meters create the indicia using a platen/ink die combination or a rotary drum/impression roller combination which are moved into contact with the mailpiece to print the indicia thereon. While traditional postage meters have performed admirably over time, they are limited by the fact that if the indicia image significantly changes, a new ink die or rotary drum will have to be produced and placed in each meter. Accordingly, newer postage meters now take advantage of modern digital printing technology to overcome the deficiencies of traditional meters. The advantage of digital printing technology is that since the digital printhead is software driven, all that is required to change an indicia image is new software. Thus, the flexibility in changing indicia images or adding customized advertising slogans is significantly increased.
Modern digital printing technology includes thermal ink jet (bubble jet), piezoelectric ink jet, thermal transfer printing, and LED and laser xerographic printing which all operate to produce images in a dot-matrix pattern. In dot-matrix ink jet printing, individual print elements in the printhead such as resistors or piezoelectric elements are either electronically stimulated or not stimulated to expel or not expel, respectively, drops of ink from a reservoir onto a substrate. By controlling the timing of the energizing of each of the individual print elements in conjunction with the relative movement between the printhead and the mailpiece, a dot-matrix pattern is produced in the visual form of the desired postage indicia image.
With regard to a postage indicia, there is a need to produce an indicia image which is visually appealing and clearly readable. The indicia image must have a relatively high optical density. That is, the density of the individual dots produced by the printhead must be sufficiently high. Moreover, it is desirable that the optical density of the indicia image is sufficient enough so that the indicia image is readable using conventional optical character reader (OCR) equipment. Furthermore, when a mailpiece having an indicia image thereon is processed by, for example, the U.S. Postal Service (USPS), it must be detected by a conventional facer/canceler machine in order to distinguish it from both stamped mailpieces and mailpieces without a stamp or indicia thereon. The facer/canceler machine typically detects a mailpiece having an indicia by exposing the printed indicia to ultraviolet lamps and then measuring the amount of radiated light emitted back by the indicia ink. If the measured radiated light exceeds a predetermined level, the mailpiece is identified as an indicia and is subsequently processed to an appropriate station for further handling. It is to be noted that in the U.S. the indicia ink is a fluorescent ink. However, in other countries the indicia ink may be a phosphorescent ink which also emits radiated light when exposed to ultraviolet lamps such that these phosphorescent indicia can also be identified by detecting the mount of radiated light emitted therefrom. Therefore, if an indicia image is to be produced digitally in a dot-matrix pattern, the density of the individual ink dots must be sufficient to allow the fluorescence (or phosphorescence) of the indicia ink to be detected by the facer/canceler as discussed above.
In producing a dot-matrix image using a digital printhead, the individual dots in the matrix are often defined according to their relative density in two directions. That is, the dots will have a certain density expressed as dots per inch (dpi)) in the direction of relative movement between the printing mechanism and the recording medium as well as a density in a direction perpendicular thereto, which perpendicular density is a function of the pitch (spacing) between each of the individual nozzles in the printhead. In the case of a very simple printhead having a single row of nozzles, the density of the dot-matrix pattern in the direction of relative movement between the printhead and the recording medium is dependent upon the speed of the relative movement between the printhead and the recording medium and the frequency at which the nozzles are energized. In the direction perpendicular to the relative movement, if a desired high dot density is required, the pitch between individual nozzles in the row of nozzles has to be precisely defined to result in the desired dot density. That is, the density of the nozzles themselves must be very high. As an alternative to using a printhead having a high nozzle density, a printhead could be used having two adjacent rows of nozzles that are offset from each other to obtain the desired dot density in the direction perpendicular to the relative movement of the printhead and recording medium. In this printhead configuration, the energizing timing of the nozzles in the two adjacent rows would have to be delayed relative to each other to allow individual columns of the indicia image to be created with the desired dot density. In yet another alternative, a plurality of printheads which are appropriately aligned could also be utilized to produce the desired dot density.
Each of the above-mentioned ways of producing the indicia image has serious limitations. With respect to using a single printhead having only a single row of nozzles, the complexity of producing a printhead which has the required nozzle density and is capable of printing the full height of the indicia image in a single pass of the printhead significantly drives up the cost of the printhead due to the complexity of manufacturing such a printhead which results in low manufacturing yields. In the case of using two adjacent rows of nozzles which are offset from each other, the manufacturing costs associated therewith is also relatively high and additional complexity is added to the meter electronics in order to control the delayed energizing of each of the nozzles in each of the rows to accurately produce the image without any noticeable shift in or misalignment of the indicia image. Finally, if a plurality of aligned printheads are used, the overall cost of the printing mechanism is obviously increased since two printheads are required versus one. Furthermore, as in the case of the adjacent rows of nozzles discussed above, the complexity of the electronics is increased to control the energizing sequence of the nozzles in the two printheads.
The Applicants of the instant invention have recognized the deficiencies associated with each of the above approaches, particularly with respect to producing a low cost postage meter for use in the home, small office, or home office environments. Accordingly, the Applicants embarked upon an approach to utilize a commercially available low cost printhead having a single row of nozzles which produces a relatively low dot density in the direction of the row. The low cost printhead produced the desired density indicia image by making multiple overlapping passes of the printhead. The printhead selected included a single row of 64 nozzles which when arranged transversely to the relative movement between the mailpiece and the printhead is capable of producing in a single pass a 0.8 inch high indicia at a resolution of 80 dots per inch (dpi) along the height of the indicia (perpendicular to the relative movement of the printhead and the mailpiece). However, since a greater dpi is desired along the height of the indicia image in order to ensure that it is detectable by a facer/canceler machine and preferably OCR readable, the printhead (or mailpiece) is shifted, after the first pass, along the height of the indicia such that during a second pass of the printhead a second indicia image identical to or substantially the same as the first indicia image is interlaced with the first indicia image to produce a combined indicia image having a density of 160 dpi along its height. Moreover, additional interlaced passes of the printhead can be performed in order to further increase the desired indicia height density, such as a third pass to produce a height density of 240 dpi. The shifting of the printhead along the indicia height is usually a fraction of the nozzle pitch, the fraction varying with the number of passes (1/2 for two passes, 1/3 for three passes, etc).
While the above solution by the Applicants allowed for the use of a low cost commercially available printhead in a postage meter for producing an indicia of an acceptable indicia height density, a potential security problem existed in that during each pass of the printhead a complete human readable indicia having an indicia height resolution of 80 dpi is produced. Thus, if three envelopes were inserted one on top of the other and then removed one at a time after each pass of the printhead, each envelope would have a readable indicia while the postage meter would only have accounted for the cost of one indicia. It is possible that despite the fact that each of these low density indicias would not be detected by the facer/canceler and would thus be appropriately routed for visual inspection by a posted worker, the quality of the indicia produced could still be mistaken as being a valid indicia during the visual inspection. Moreover, depending upon the density of the image produced during the three passes, it was also possible that each of the three images would be identified by a facer/canceler machine as a valid indicia.
In view of the above, the Applicants recognized that a more secure way of printing a desired density indicia is required which would still permit the use of commercially available low cost/low density printheads. The instant invention is directed toward the method and apparatus associated therewith.